The present invention relates generally to internal combustion engines, and more particularly to a compression release mechanism which operates a valve at low engine speeds to relieve compression within one or more engine cylinders during the compression stroke of the engine cycle.
It is often desirable to relieve the compression in an engine combustion chamber during starting in order to reduce the force required to turn over the engine. By relieving this compression, it is much easier for the piston to reciprocate in the engine when the operator manually pulls the starter rope. Thus, a compression release mechanism lessens the pull force required to start the engine. As a result, the potential for operator fatigue during manual starting is also minimized.
Compression relief apparatus of the centrifugally-responsive type are disclosed in U.S. Pat. No. 3,381,676 issued May 7, 1968 to Campen, No. 4,453,507 issued Jun. 12, 1984 to Braun, et. al., No. 4,892,068 issued Jan. 9, 1990 to Coughlin, No. 4,898,133 issued Feb. 6, 1990 to Bader, No. 4,977,868 issued Dec. 18, 1990 to Holschuh, No. 5,184,586 issued Feb. 9, 1993 to Buchholz, and No. 5,197,422 issued Mar. 30, 1993 to Oleksy et. al. These patents reveal a variety of designs for centrifugally-responsive compression release mechanisms for internal combustion engines.
The Campen '676 compression release mechanism is typical of the prior art. Campen '676 includes a centrifugally-responsive flyweight, a torsion spring attached to the flyweight, and a central pin member therein which engages a valve tappet at engine starting speeds. At higher engine speeds, the flyweight moves radially outward so that the pin disengages the valve tappet when the engine is running.
A major disadvantage of many prior centrifugally-responsive compression release devices is that the design often requires a major modification to engine components or additional parts in order to attain the requisite stability and reliability. These modifications present an undesirable burden on the assembly process and on component manufacturing. As a result, the overall cost for the engine and for its maintenance is increased.
In the Campen apparatus, for example, the cam shaft must be modified prior to assembly to include the central pin member therein. Also, the shaft about which the flyweight rotates must be fastened to the flyweight, resulting in additional complexity and expense. In Coughlin '068, the flyweight must be attached to the timing gear by a rivet during assembly. In Holschuh '868, the cam lobes on the cam shaft must be machined for assembly of the compression release device.
The compression release apparatus in Buchholz '586 illustrates another problem. The Buchholz '586 compression release apparatus is disposed on the outboard side of the outboard cam lobe. Since the components of the compression release apparatus encircle the cam shaft, these components must be specially machined to accommodate the cam shaft prior to assembly. Furthermore, the apparatus must be installed on the cam shaft in conjunction with the assembly of the other engine components that are mounted on the cam shaft.
The problems of prior art compression release mechanisms are further magnified when it is desired to relieve compression in more than one combustion chamber of an engine. Foremost, the foregoing designs are not readily adaptable to operating a plurality of valves, such as the exhaust valves of a V-twin engine. To incorporate the prior art designs on such an engine would require a duplication of parts, and a significant increase in weight and complexity.
In general, the centrifugally-responsive compression release mechanisms of the prior art are either too complex, employ too many parts, or require a major modification to the engine. Moreover, many of these devices place too great a burden on assembly and manufacturing, thereby increasing cost further.